Derby — Occurrence of Topaz near Oaro Preto, Brazil. 29 



meters thick of micaceous material that still retains a lithoid 

 character and is composed almost exclusively of fine flakes of 

 muscovite, which also occurs in considerable abundance and in 

 larger flakes in the earthy and quartzose matter of the nodules. 

 Washings of the various phases of the topaz-bearing earth 

 give a very abundant slime of earthy iron and manganese oxides 

 and finely divided mica with a greater or less amount that is 

 difficult to estimate of clay matter, leaving a comparatively 

 insignificant residue of mica flakes and fine grains of scaly 



hematite. The normal earth free from visible kaolin of the 

 specimen of fig. 2 gave 27'8 per cent soluble in hydrochloric 

 acid, of which 3 per cent was manganese oxide, the rest being 

 iron with a very perceptible trace of alumina. The darker 

 earth of the brown clay nodules and that surrounding one of 

 quartz gave about twice as much soluble constituents, that is to 

 say of iron and manganese oxides. The residue after sliming 

 is nearly free from quartz except when quartzose nodules occur. 

 On removing the mica and iron a very slight heavy residue of 

 minute grains and aggregates of rutile and needles of tourmaline 

 remains with very rare and ill-formed grains of green anatase 

 and a comparative abundance of a yellowish phosphatic 

 mineral. The hematite grains are rare in the body of the 

 earth but very abundant when the micaceous crusts above 

 mentioned occur, and the tourmaline is distributed in a similar 

 manner but is nowhere abundant. Kutile appears in all the 

 washings, but is apparently more abundant in the main body 



